Conventional fuse structures that can be modified with a laser are common components in Built In Self Repair (BISR) circuits used in memory devices. Referring to FIG. 1, an example of a circuit 10 illustrating a conventional fuse structure is shown. The circuit 10 includes a fuse 12, a latch circuit 16, a transistor D1, a transistor D2 and a scannable flip-flop 18. Normally the latch circuit 16 is held to a known state by the transistor D1. The transistor D2 is used to pass the state of the fuse 12 to the latch circuit 16. If the fuse 12 is intact, the input to the latch 16 is effectively tied to VSS, resulting in a high state on the output of the latch 16. If the fuse 12 is blown, the input to the latch 16 will be floating, resulting in a continued low output from the latch. The state of the fuse 12 is captured by the scannable flip-flop 18. The flip-flop 18 is connected to other fuse structures (not shown) in a fuse scan chain. The state of the fuse 12 is often difficult to diagnose, particularly after the fuse 12 has blown prior to final testing.
The inability to diagnose fuses in independent fuse structures may be partially overcome if none of the fuses have blown. The latch circuit 16 is normally in a low state because of the pull-down transistor D2. By scanning the initial state of the fuse 12 with the flip-flop 18, and then capturing and scanning the opposite state output of the fuse 12, some diagnostic capability is provided. The initial state at the D input of the flip-flop 18 is low prior to activation of the signal READ. Capturing and scanning the initial state allows for detection and diagnosis of defects in the scan chain that results in a stuck-at high condition. After activating the signal READ, the state will switch to high (prior to blowing the fuse 12) and defects in the scan chain that result in a stuck-at low condition can be detected. After the fuse 12 has been blown, it is not possible to detect stuck-at low defects in the scan chain with such a conventional approach.
Such a conventional approach will not work if one or more of the fuses in any of the fuse circuits have blown. The alternative technique described is also only useful for identifying defects that exist in the fuse scan chain. Additionally, the alternative technique does not assist in detecting defects that affect the latch circuit 16 and/or the flip-flop 18.
It would be desirable to provide a method and/or apparatus to (i) allow diagnostic capability of a fuse circuit when a fuse has blown and/or (ii) assist in detecting defects that affect the latch circuit and/or the flip-flop.